The ephrins are membrane-bound proteins that act as ligands for the Eph Family of receptor tyrosine kinases. Ephrins and Eph receptors are involved in angiogenesis, axon guidance and topographic map formation, and segmentation. Evidence suggest that ephrin B2 and its receptor Eph B4, are the first molecular markers that distinguish arteries from veins, respectively, and these molecules are involved in the earliest stages of angiogenic remodeling of the primary capillary network established by vasculogenesis. Our long term objective is to obtain the structural basis for the function of the ephrins and the Eph receptors using X-ray crystallography. This information is critical for understanding the principles of angiogenesis and the relevant diseases such as cancer, eye diseases, and ischemia. Current studies on ephrins and Eph receptors focus on two key questions: 1) How ephrin extracellular domains cluster to activate Eph receptors. 2) What is the basis of the promiscuity of binding between Eph receptors and ephrins that is observed in vitro, which suggest a range of specificity that must be present in vivo. To address theses problems we propose to solve the X- ray crystal structure of ephrin B2 extracellular domain, for which we have already obtained well-diffracting crystals. We expect to determine the nature of ligand oligomerization, and see how the ligand might be bound by Eph receptors. The high- resolution, three-dimensional structure of ephrin B2 will allow us to design epitopes for monoclonal antibodies, and allow future directed-drug design, both of which could have therapeutic applications. Further, we propose to address the structural basis for Eph-ephrin binding by crystallizing and solving X-ray crystal structures of complexes of ephrin-Eph ligand binding domains. Finally, we propose to investigate the relationship between structure and function of the entire Eph receptor ectodomains via crystallography, to ascertain the function of the non-ligand binding domains.